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The first tube stage is configured in a pretty standard way. I didn't hear enough difference between 100K and 220K plate resistors to want to add a switch for this option, and went with 100K. A on-off-on SPDT switch adds resistors in parallel to a 2K7 cathode resistor. This provides 3 choices of cathode resistance--2K7, 1K5, and 820 ohms. Switching the resistance this way is noisey--it pops--but I'm not thinking of these settings as something that are fiddled with in the middle of a song. I thought this stage sounded best with a cathode bypass capacitor, so a 1uf cap is always in the circuit. The output of this stage can either be directed to a volume control feeding the output jack, or on to other tube stages.
If routed to more tube stages, switch S2 determines if the 2nd tube stage is bypassed or included. The choices for cathode resistance on the 2nd stage are 10K, 3K3, and 2K8. In addition, a cathode bypass cap of 0.68uf can be switched in.
Tube stage 3 has two choices for cathode resistance: 820 and 3K3, and can have a 0.47uf cap switched in.
From there, a cathode follower provides a low impedance signal for the tone stack. The tone stack was designed using some really nice simulation software. Standard bass, middle and treble controls are available. The signal then goes on to a master volume and out to a power amp.
The power supply was inheirited from the original microphone preamp. None of my design is in there, but I think I may poke at it a bit. Although the preamp is relatively quiet (in the low gain modes), I think full-wave rectification might make it quieter.
UPDATE: August 1999
A full-wave bridge rectifier does reduce the hum noticeably, as well as raise the voltage. As I was dialing in the single tube stage in front of a Marshall, I became aware that the hum really was annoying. The bridge cured the problem.
The power transformer does not have a center tap, so I needed to use a full wave bridge (4 diodes). The AC connects to where it normally would, you ground the (-) output and use the (+) output to feed the filter capacitors.
This project started out as a way to add an additional tube stage to an amp without modifying the amp. I wanted to play with making a Fender sound like a Mesa Boogie and an old Marshall sound like a modern one. I had an old microphone preamp as a starting point, and it used 2-12AX7 tubes. I only needed one additional tube stage for the above, but rather than ignoring the potential of the extra stages, I added some features to put them to use.
The final design allows you to add a tube stage to an amp. In addition, you can switch in one or two more tube stages that connect to a cathode follower driving a tone stack. I was interested in the role of the cathode resistor and bypass capacitor values, so there are switchable combinations. By setting toggle switches, you can configure the preamp to be similar to a Fender 5F6 Bassman, the dark or bright channel of a Marshall Super Lead, or some of the Marshall JCM800 models. I'm thinking that with the addition of a low-impedance output jack, this preamp will be a backup for playing out. If my main amp goes, I'll run this into the PA and be able to get reasonable guitar tone.
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